This invention concerns a continuous casting method with a magnetic field and the relative device.
The invention is applied to machines performing continuous casting of billets, blooms and slabs and, in particular, thin slabs in the field of the production of iron and steel.
The state of the art of the continuous casting field covers the use of electromagnetic devices associated externally with the sidewalls of a crystalliser and able to generate an electromagnetic field interacting with the molten metal being cast.
In the state of the art this electromagnetic field mainly has the purpose of improving the surface quality of the product and/or of increasing the casting speed by taking action on the parameters of formation of the layer of solid skin and by causing to happen earlier a separation of the skin from the sidewalls of the crystalliser; another purpose is to displace the surface of the molten metal in the zone of the joint between the refractory material and the crystalliser so that the solidification begins only in the crystalliser and there are no leakages of material.
The electromagnetic devices of the state of the art normally comprise a coil or one single inductor positioned in cooperation with the outside of the wall of the crystalliser and generally close to the zone of the beginning of solidification of the metal.
Embodiments have been disclosed in which the coil or inductor generates a stationary alternating magnetic field (see the article "Improvement of Surface Quality of Steel by Electromagnetic Mold" taken from the documents of the International Symposium on the "Electromagnetic Processing of Materials"--Nagoya 1994) or else generates an alternating magnetic field modulated in amplitude (see the article "Study of Meniscus Behavior and Surface Properties During Casting in a High-Frequencies Magnetic Field" taken from "Metallurgical and Materials Transaction"--Vol.26B, April 1995).
Other embodiments disclosed provide for the magnetic field generated to be periodically pulsating with waves defined by successions of pulses of a substantially constant amplitude (U.S. Pat. No. 4,522,249) or else for the magnetic field to be generated by electromagnetic waves of a development which is attenuated until it is eliminated within a half-period (SU-A-1021070 and SU-A-1185731).
To be more precise, the teaching disclosed in U.S. Pat. No. 4,522,249 includes a helical coil wound around the crystalliser along its whole lengthwise extent.
This helical coil is fed by means of a pulsating direct current of from 10 to 100 Ms, an amplitude of between 5 and 20 kA, a frequency of repetition of around 1 KH.sub.Z. The current generates radial forces which act on the crystalliser in order to make it vibrate. The vibration serves to eliminate the mechanical oscillation and tends to improve the surface quality of the product.
The action of vibration induced on the crystalliser may cause, and indeed does cause, breakages due to fatigue; moreover, the vibration is not able to act on the product with actions of the migrating field type or multi-modal excitations, which are those that obtain an effective usable result.
WO-A-80/01999 and FR-A-2.632.549 include electromagnetic devices consisting of radially arranged poles on which the coils are wound; the devices are arranged at different levels and are made to function in a staggered manner.
The coils are fed with alternate current, low frequency mono-phase or multi-phase, and they generate forces which are mainly directed in an azimuthal direction and only by reflection in a lengthwise direction along the axis of the crystalliser.
These electromagnetic devices have the function of mixing in an azimuthal direction the liquid steel in the crystalliser in such a way as to produce a helical motion either upwards or downwards.
U.S. Pat. No. 4,933,005 includes permanent coils or magnets operating both in correspondence with the meniscus and in a desired zone of the crystalliser. The coils arranged along the crystalliser, and far from the meniscus, generate mainly azimuthal forces (azimuthal stirring) or helical forces (helical stirring) or longitudinal forces (longitudinal stirring); the coils arranged in correspondence with the meniscus generate forces which oppose the movement of the liquid part of the product.
The coils placed far from the meniscus serve to move the liquid part of the product so as to obtain the known metallurgical results deriving from electromagnetic stirring. The coils which cooperate with the meniscus serve as an electromagnetic brake in order to reduce the consequential distorsions caused to the meniscus by the electromagnetic stirring generated by the other coils, and also to reduce the turbulence caused by the introduction of material into the crystalliser.
EP-A-0.511.465 discloses a coil for electromagnetic stirring which can be displaced along the axis of the crystalliser, in such a way that it is possible to adapt the electromagnetic stirring effect in the liquid metal according to the different metallurgical requirements.
EP-A-0.489.202 provides for coils which cooperate with the crystalliser and fed with direct current; they generate a constant magnetic field with the appropriate direction. These coils serve to brake the liquid steel which leaves the submerged discharge nozzle so as to prevent the scouring of the already solidified skin and at the same time to reduce the trapping of the slag.
U.S. Pat. No. 4,867,786 and JP-A-56.126.048 provide for coils which produce azimuthal flows so as to mix the liquid part of the metal with a stirring effect in an azimuthal direction, in order to obtain the desired stirring effects.
WO-A-94.15739 discloses two traditional coils for electromagnetic stirring, of which one is located on the meniscus.
Both coils are fed with low frequency, multi-phase alternating current, possibly with different intensities of current; the direction of the magnetic field migrating over the pole pieces may also be different.
The forces generated are applied on the liquid part of the product in an azimuthal direction.
The function of the underlying coil is to provide for the azimuthal stirring of maximum intensity; the function of the coil on the meniscus is to contrast the distorsion produced on the meniscus by the stirring effected by the first coil or, alternatively, to increase the effect on the meniscus according to the particular type of process or the type of casting (type of steel).
Experimental tests have shown that the configurations of the electromagnetic field acting in the crystalliser, in the state of the art as described above, are not suitable to achieve the results desired by the Proprietor of this invention, in view of the different conditions which take place within the solidifying metal.
These different conditions, which are due to the different physical state and different temperature of the solidifying metal, cause an interaction between the magnetic field and the metal, this interaction being different from one zone to another of the crystalliser and therefore not being the best along the whole length of the crystalliser. In particular, but not only, the state of the art does not allow to fulfil the following functions in a positive manner:
to reduce the friction between the cast product and the crystalliser by inducing pulsating forces directly onto the solid skin of the product, and also onto the liquid part where that is necessary, in order to increase the casting speed; PA1 not to use the traditional methods of mechanical oscillation of the ingot mold, with a consequent improvement of the surface quality of the product, as the oscillation marks are eliminated; PA1 to control the effect on the meniscus according to the requirements of processing, so as to improve both the lubrification of the area of contact between the skin and the sidewall of the crystalliser, and also the surface quality and the inner quality of the product; PA1 to use the capacity of resonance of the solidified skin and the skin-liquid system, so as to improve the heat exchange performed in the mushy zone in order to encourage a growth of the product with an equal axis, and a consequent improvement in the inner quality; PA1 to use the migrating field configuration in order to induce, in the liquid part, a vertical stirring (direction of the axis of the crystalliser) so as to obtain an optimum effect; PA1 to improve the heat exchange in the lower part of the crystalliser where the skin is separated from the crystalliser, thus increasing the total quantity of heat extracted by the crystalliser and making it possible to achieve higher casting speeds and improvements in the quality of the product. PA1 to reduce the friction between the cast product and the crystalliser by inducing pulsating forces directly onto the solid skin of the product, and onto the liquid part where that is necessary, in order to increase the casting speed; PA1 not to use the traditional systems of mechanical oscillation of the ingot mold, and therefore the crystalliser, with a consequent improvement in the surface quality of the product as the oscillation marks are eliminated; PA1 to control the effect on the meniscus according to the requirements of processing, so as to improve both the lubrification and the surface and inner quality of the product; PA1 to exploit the capacity of resonance of the solidified skin and the skin-liquid system, in order to improve the heat exchange in the mushy zone so as to encourage a growth of the product with an equal axis and a consequent improvement in the inner quality of the continuously cast product; PA1 to use the migrating field configuration in order to induce in the liquid part a vertical stirring (direction of the axis of the crystalliser) so as to obtain an optimum result in the cast product; PA1 to improve the heat exchange in the lower part of the crystalliser where the skin is separated from the crystalliser, thus increasing the total quantity of heat extracted by the crystalliser and making it possible to achieve greater casting speeds and at the same time to improve the quality of the product.